Cognitive radio refers to a radio device which can intelligently detect which communication channels are in use by primary users and which are not, and move into vacant channels while avoiding occupied ones.
An important function of cognitive radios is spectrum sensing, generally referring to detecting unused spectrum. Cognitive radios attempt to sense “spectrum holes”. A spectrum hole is a frequency range, where the absence of primary users has been determined. Primary users are for example TV stations, wireless microphones or cellular systems. Detecting the absence of primary users is a difficult task and a single detector may make errors when determining whether the spectrum is primary free or not. Efficient spectrum use in the cognitive radio bands also requires coexistence between secondary users.
In passive coexistence methods a cognitive radio selects an operation frequency or transmission time based on sensing the channel. Active coexistence, where cognitive radios can share the characteristics and sensing information with other cognitive radios, is expected to enable more efficient spectrum use than passive coexistence methods.